Proceedings of The IEEE International Conference on Industrial Technology, 1996

MODELLING AND SIMULATION OF DIESEL ELECTRICAL AGGREGATE VOLTAGE CONTROLLER WITH CURRENT SINK

G. Erceg, IEEE Member Department of Electric Machines and Automation Faculty of’Electrica1 Engineering and Computing

University of Zagreb, Zagreb, Croatia

Absiract - In the paper modeling and simulation of diesel motor with speed control system and generator with Voltage system as complex, nonlinear and multivariable systems are presented. Mathematical models for every part of diesel electrical aggregate (diesel motor with its speed control system, synchronous generator, synchronous exciter, self-excited compounding unit, voltage controller and load) are developed and connected in common model of aggregate. Upon these mathematical models of the aggregate, simulation model is made using “MatrixX)) simulation software package.

The paper also deals with comparation of characteristics seen from the simulation aspect and measurements. Bringing into a model new current sink, simulation results better match experimental results. So, this model is privileged for selection of diesel electrical aggregate’s type and for analyzing quality of load supplying. Keywords - Diesel Aggregate, Modeling, Sunulation, Isolated System.

I. INTRODUCTION

Diesel electrical aggregates usually supplies isolated systems, or acts as a reserve power suppliers. Such systems are characterized with the fact that aggregate’s power fits almost equally the power of installed loads. Procedure of switching bigger loads in such systems causes considerable dynamic changes in amplitude and settling time of voltage and frequency. Considering quality of supplied load, known characteristic appeared in transient process during loading and/or unloading of aggregate result with proper choice of diesel electrical aggregate’s type.

Investigation of diesel electrical aggregates behavior requires usage of modern techniques in mathematical modeling and simulation studies, and also carrying out of measurements.

11. MATHEMATICAL MODEL OF DIESEL ELECTRICAL AGGREGATE

Principle scheme of diesel electrical aggregate is shown in Fig. 1. Aggregate is equipped with generator wltage control system and diesel motor speed governing system. Generator excitation system is realized using brushless synchronous exciter and compounding unit. Besides the main generator

S. TeSnjak, CIGRl? Member R. Erceg, EEE Student Member

Department of Electric Power Engineering Faculty of Electrical Engineering and Computing

University of Zagreb, Zagreb Croatia

voltage feedback loop, the internal feedback loop is provided using excitation current of exciter. . Selfexcited compounding unit operates as a current supplier, always giving more current than the excitation of the exciter needs. Generator voltage control system is based upon performs on principle of subtraction of exciter excitation current using current sink. Voltage controller has PID characteristic, while exciter excitation current controller has P characteristic. Pulse-width modulator (PWM) operates with current of current sink, commonly known as exciter excitation current. Initial excitation is realized via self-excited resonant LC unit, while compoundation via compound transformer. Generator frequency and resonant frequency of LC unit are equal.

In control system of generator voltage new current sink (Fig. 2) with reactor and transistor switch is used. This current sink is capable of achieve changes of exciter excitation current in much wider range (practically from zero to maxim-) with less loading current of transistor than today’s current sinks with resistor and transistor switch. Transistor switch needs constant frequency and‘ changeable transistor impulse conduction period. Characteristic of exciter excitation current which depends on PWM operation

4 U, r - - - - - - - - - - - - - I

I

1 - measure unit 2 - voltage controller (PIC)) 3 - controller of exciter excitatlon current (P) 4 - pulse-width modulotor (PWM) 5 - diesel motor 6 - synchronous generotor 7 -_ brushless generotor 8 - three phose diode rectifier 9 - meosure unit of exclter excitatlon current

10 - pulse omplifier 1 1 - current sink

Fig. 1 Principle scheme of diesel electrical aggregate

0-7803-3 104-4 . a75 -

“0 “b “c w

GENERATOR

Fig. 2 h c i p l e scheme of self-excited umt with current sink

signal is nonlinear and also depends on frequency. Linear static characteristic of exciter excitation current with internal control loop is taken enabling increase in speed of system performance.

Diesel motor accompanied with control system and generator with the voltage control system make complex, nonlinear and multivariable systems. Mathematical models are developed for each part of diesel electrical aggregate (chesel motor with speed control system, synchronous generator, brushless exciter, self-excited compounding unit, vdltage controller and load), whch are afterwards ccinnected in general model of the aggregate.

A. Diesel motor

Diesel motor expresses characteristics of complex, multivariable and nonlinear system. Its dynamic behavior is presented via system of differential equations of motor, turbocompressor, inlet and outlet collectors and speed controller. By taking into consideration of dfierentid equations of turbocompressor, inlet and outlet collectors, khowledge o€ many diesel motor characteristics are required as much as very complicated experimental measurements. Because of observing priority of generator’s characteristic variables, diesel motor as the part of the aggregate is in transient process usually given with simplified model. Thus, neglected differential equations of turbocompressor, inlet and outlet collectors results with simplified functional scheme (Fig. 3).

It is assumed that speed control has direct impact on opening of gas leading valve “a” which fits the quantity of motor gas leading in no-load state and transient process. Also, in diesel motor operation, the torque pulsation is neglected assuming direct proportion of torque (without delaying) to quantity of motor gas leading.

Speed controller has different designs. In today’s practice, direct mechanical? mechanic-hydraulical, elecrohydraulical and electronic controllers with microprocessors are used. In the paper, motor with direct mechanical controller is taken.

/ I

Fig. 3 Simplified functional scheme of diesel motor

Dynamic performance of diesel molDr with its speed

(1)

control system c m be described by following equations: da,

J D M ‘-z = TDM - T& >

where JDM is aggregate inertia torque, TDM diesel motor torque, Teh generator’s electromagnetic torque, z1 and z2 time constants of speed controller, w aggregate speed, coo reference aggregate speed, k control amplifier and TDMo diesel motor torque at no-load state.

B. Synchronous generator

Usually, mathematical models or synchronous generator are given by transforming steady state abc coordinate system into dq coordinate system. According to references, mathematical model is given in dq system with nonlinear variables, but the way of taking nonlinearities into the model depends on solving the concrete problem. Because of saturation in mathematical model of generator with self- excitation remanence and nonlinearities using no-load state characterislics. In the model of the generator, changes of variables are taken in d-axis. It is supposed that variables in q-axis are constant. The p.u. equations of mathematical model of generator with saturation included are: - vd = R, . id + ( x i . id + xJd . ir + x& ‘ 1, + ydO) + w . y, , (3.a)

(3 .b)

( 3 4

(3.4

(33)

(3.0

Changes of variables in d-axis of generhtor model which are taken in no-load state condition are marked with *, while index “0” denotes remanence. Change of saturation caused by generator speed changing is neglected. In Fig. 4 change of reactance xid is shown. Supposing that dissipation of reactances are constant, x;d = x& = x& worth.

- v , = R , ’ l , + S ( X , ‘ l , + X g * i Q ) - ~ ’ v / d ,

VI = R, .i, f s x M . id +x; .if +x&. io +yll,,) , ( ‘ ( *

0 = R, .i, +s ( xa . ‘ zq +x, .iQ ) , T, = id (x, . I , + x g . i, ) - ( . . ; . I d + x ; .t,+x; .i, +

0 = R, .io +s xdD . id +x& . i f + x i . i , +yo,,) ,

+ v D 0 ) ‘ ‘ q

C. Brushless exciter with rotating diode rectifier

Presently, according to references full dynamic performance of brushless exciter with rotating diode rectifier is not explored in depth in different generator operating

- 1 - 1 I

, . . . . . . , . . .

0 0 1 0.4 0 1 0.1 I I1

Fig. 4 Changes of reactance xi. in d-axis of generator

modes. Internal resistance, exciter armature reaction and commutation process of &ode rectifier have considerable effects on static and dynamic characteristics of exciter. By all electromagnetic effects with speed change taken into consideration of brushless exciter with rotating diode rectifier, from the complex mathematical description results which is not appropriate in computation. Model (Fig. 5 ) is chosen from [2] where brushless synchronous exciter is modeled with saturation (SE) at no-load state, and diode rectifier with valve characteristic (Fe). Effect of armature reaction is represented via kD factor. Commutation reaction of diode rectifier is defined using subtransient reactance of exciter.

D. Mathematical model of load

Transformation of voltage equations of load from abc coordinate system into dq coordinate system results with mathematical model of generator's load which is stated in p.u. as follows:

(4)

( 5 )

V, = R , . id + s . x L . id + w . x , . i, , V, = R L . i q + s . x , . i , - w . x , . i , .

E. Self-excited resonant compounding unit and exciter excitation current control loop

Self-excited compounding unit posses ''mixed" (AC-DC) current circuit, and 111 mathematical descriptions would lead to vexy complicated simulation model. Simplified model is achieved through compensation scheme (Fig. 6) where mixed circuit of self-excited unit is replaced with AC circuit. In compensation scheme generator is equipped with voltage supplier, compound transformer with current supplier, and winding of exciter excitation with equivalent resistance of exciter excitation.

Upon this basis of compensation scheme with transformation of abc coordinate system into dq coordinate system, the self-bxcited resonant compounding unit is modeled with equations as follows:

IQ I 'f

Fig. 5 Structural diagram of synchronous exciter with rotating diode rectifier

A 1 1 I

Fig. 6 Compensation scheme of self-excited compounding unit

0, x , . i,, + Vc, , V, = R, * i,, + x , - - 4, dt

PWM is in operation at frequency of 330 Hz, and can be simplified in modeling of practical operations. Such model of P W M is not suitable as a part of common model of the aggregate. Starting from the fact that self-excited compounding unit determines exciter excitation current, needed correction of exciter excitation current is made by voltage controller. Control loop of exciter excitation current with self-excited unit is modeled as it is shown by structural block diagram (Fig. 7).

F. Generator voltage controller

Generator voltage controller has PID characteristic. Its mathematical model with feedback loop elements included is depicted by structural scheme (Fig. 8).

111. SIMULATED AND EXPERIMENTALLY OBTAINED RESPONSES

For each part of the system, the simulation model is made by using ''Matrix? simulation software package. Different parts of the simulation model are connected into common model of the aggregate (Fig. 9). Dynamic behavior of the aggregate is simulated regarding nominal step loading andor unloading of generator (54 kVA, 400 V, 78 A, 1500 mid', 50 Hz, cosq~0.8) as a triggering events.. Simulated and experimentally obtained responses of the aggregate (Fig. 10 and 11) match in significant degree approving diesel electrical aggregate model correction.

Fig. 7 Structural block diagram of exciter excitation current control loop

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................................... ......................................... ~

1.2 , , , I , , . , . , . , , , ,

. . . . . . . . . . . . . . . . , , , . , . , . , , .

. . , . .

1 - : : < " ' ; ; ; : : : ; : : . . . .

0.8 ............. 4 ...... L ..... : ..... : ..... ...... ..... :.. .......... I ................. .................. . . , . . , .

. . . . ................. , ................... . . . . .

. . . . , . . o I . . . , . . . .............. ... ... ........... ..... ........ ........ ... ..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

, . . . . . .

, , . . . 0.2 :. ..;. .;... .;. ; <... ..<.. .-.:.. .<. .;. .,

o . ' : ' ! ' ! ' : ' : ' : ' ! '

. .

CONTROL LOOP

1 . 2 - . . . . . . . . . . . . . . . - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . ....... 1 . ' : I ;-.Cy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0,: :......:.....:.....:.....A ..... ; ..... ; ..... ; ..... .. ._.... ;.....;.....;...I.. ; ..... ; .....

. . . . . . . . . . . . . . . . . . . . . . , . , . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.2 .____: ...... 1 ..... :.. ..: 1 :.. ..: _. : L i : i ..... ...... ..... ...... ..... ..... ..... ...... ...........

! .

o - ' ! ' ! ' : ' ! ' ! ' ! I ! '

I I

11 +sT2 Fig. 9 Matrix, System Build simulation model of diesel electrical aggregate Fig. 8 Structural block diagram of generator voltage controller

1.2

I

3 * 0.1

0.6

f 0.1

0.2

0 0 0.1 0.1 0.3 0.4 0.5 0.6 0.7 0.1

The (4

I .f

1

0.8

::: 0.2

0

0 0.1 0.2 0.3 0.4 0.1 0.6 0.7 0.8

111. (s)

0 0 1 0 2 0 3 0 4 0 3 0 6 0 7 0 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8

T h e (s) Tim. I.)

a) Simulation responses b) Expeiimentally obtained responses

Fig. 10 Responses of loading diesel electrical aggregate

I?.

I - * 0 8

g $ 0 6

5 0 4

0 . 2

0

... .... I - -

0.8 ..... .... - $

1 . 1 - ,

, 0.8

. I . . . ;i

0 0.1 0.2 0.3 0.4 0.5 0.68 0.7 0.8

Tlmcls)

. . . . . , , . , , , , , , . , , , . , , . , . I . . . . . . . . . . . . . . . . . , . . , , . , . . . . . . . . . . .

......... . . . . . . . . . ”.. . , . . . . . . . . . . . . . . . . , . . ) . . . . . . . . , . ) , . . . . . . . . . . . . . . . . . . . .

: : : :-- , ,

....... : ............ L ..... ; ..... : ..... : ...... : ..... : ..... : ...... ; ..... L ..... ;.....:......I ..... : .... . . . . I , , , . . , . , , , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..... ...... ..... ..... ..... .... ..... ..... ...... ..... ..... ...... ..... ....

I ,. x 0.8

0.6

! 0.4

0 1

0

0 0 1 0 2 0.3 0.4 0.5 0.6 0.7 0.8 Tlma (I) Tlme (r)

IV. CONCLUSION

Results of diesel electrical aggregate simulation matches experimentally obtained results in signifkant degree introducing this simplified model as a useful mean in practical purposes.

Linear and nonlinear variables of diesel electrical aggregate components have to be more correctly given or measured. Their uncorrection can cause considerable changes between simulated and experimentally obtained responses.

This model of the aggregate gives opportunity of exploring diesel motor speed control system effects on generator voltage control system performance.

V. REFERENCES

1.2

1 m d

f o’8 I 0.6

f 0.4

0.2

0

d

0 0.1 0.2 0.3 0.4 0.5 0.6 0 1 0.8

T h e ( 8 )

o 0.1 0.2 0.3 0.4 0.5 0.6 0.1 0.8

8

[ 3 ] S. Stavrakakis, G. N. Kariniotakis, “A General Simulation Algorithm for the Accurate Assessment of Isolated Diesel - Wind Turbines System Interaction”, Part I: “A General Multimachine Power System Model”, IEEE Trans. on Energy Conversion, Vol. 10, No. 3 , pp. 577-583, Sept. 1995.

[4] S. Stavrakakis, G. N. Kariniotakis, “A General Simulation Algorithm for the Accurate Assessment of Isolated Diesel - Wind Turbines System Interaction”, Part 11: “Implementation of the Algorithm and Case- Studies with Induction Generators”, IEEE Trans. on Energy Conversion, Vol. 10, No. 3, pp. 584-590, Sept. 1995.

[ 5 ] “Matrix, - SystemEIuildNS’, Version 3.0, Integrated Systems Inc., USA, 1992.

[l] Anderson, A. A. Fouad, “Power System Control and Stability”, IEEE PRESS Power System Engineering Series, USA, New York, 1993.

[2] IEC Technical Report 34-16-2, 1991.